Creative EP-830 Headphones: Right Earpiece Volume Decrease - Possible Burnt Converter?

The vast majority of this type of headphones are non-dismountable designs. Precisely because of the miniaturization and the need to maintain an aesthetic appearance.
In such cases, the service under warranty will replace them with new ones.
The only thing you can check is whether they are not "clogged" with earwax (contrary to appearances, it is quite common - especially when someone listens for several hours a day).
If there are traces of earwax on the safety strainer (or some other dirt), you can try to rinse (rinse gently) at WL CONTACT (IPA).
After the treatment, dry the headphones thoroughly and only then check if it has improved.
If the damage is only due to quieter playing, and you do not hear any distortions, buzzing or a clear change in sound (the lack of bass), it is possible that the filter is clogged. If you hear clear distortions - the earpiece itself (the coil or the diaphragm) may have been damaged - sometimes it happens that despite the strainer something falls inside (mostly dust is the reason behind it) and partially immobilizes the transducer diaphragm - but then you can also hear distortions.
The cable is rather okay - you would notice changes in volume or crackles when it is bent (usually it breaks in the most vulnerable places - either at the plug or when exiting the bend at the headphones).
So - if washing does not bring a result, the only thing left to do is replace the headphones with new ones ...